So just as I suspected; Arka and Memnos indeed do have a completely different outcome after their encounter with Sazza and both are actually intended to be present for the Grove siege.

Here is Arka on the ramparts ready for the fight.
[Linked Image from i.imgur.com]

And here is Memnos, who can be found praying at the Grove's centre, but can be persuaded to help with the battle.
[Linked Image from i.imgur.com]

And here they both are saying their final goodbyes to Kanon afterwards.
[Linked Image from i.imgur.com]

This is why I mentioned;

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Which makes this whole story arc so disappointing and the main reason why I completely avoid Sazza's quest in majority of my playthroughs. Because Arka's corpse otherwise stands as a permanent monument of failure and a reminder that she is dead because of me.

Because triggering their encounter with Sazza causes their disappointingly unavoidable deaths completely devoid of player choices or actual meaning, so made me feel responsible every time I triggered it. However by ignoring the encounter completely until the siege begins, Sazza dies and they both live and actually get the encore they're supposed to get.

So I'm quite happy knowing that Arka's story has a proper conclusion and that there is a very big chunk of it missing in-between the events. I'm sure the encounter will be properly fleshed out eventually, it was just very strange to me how it presented itself as a whole story.